Muscovite

As a child, I often remember seeing shiny bits of material in the dirt in our back yard. My father told me it was called mica and that his employer used it in the products they made. Occasionally, we found pieces that had a whitish look to them, but most were a yellowish-brown color and sparkled like gold or silver. Sometimes lighting conditions can make a difference in color, as all of the photos in this post are of a single specimen I collected from the back yard very recently. When held in shadow and reflecting diffused light, the sample appears silver-colored, but in the other shots where the sample is held in light rather than shadow, the true yellowish-brown shows clearly.

So, what is mica...? It is a silicon-based mineral that has "varying amounts of aluminum, potassium, magnesium, iron and water." ^[2] There are several types of mica, but the most common are: muscovite, phlogopite, biotite, and lepidolite. I don't have any biotite in my mineral collection, but I have the other three. Besides these big four, there's also paragonite, illite, phengite, and a few others.

This specimen is muscovite, which is the most common form of mica found on the earth. Muscovite has the ability to resist heat, so it is used commercially in fireproofing materials. ^[4] It also has the ability to insulate against electrical current, and that is how my father's employer used it, in the manufacture of capacitors for use in electrical circuits. Muscovite has been found in India in sheets 5 meters × 3 meters (16.5 feet × 10 feet) in size. ^[4]

The piece I am holding, below, is typical of the size found in this area, but I suspect that larger pieces might be found in the mountains nearby where no land development has taken place. Muscovite is shiny, like a mirror, and reflects light easily. In the following shot, the mica is in sunlight but pointed away from the sun:

And below, it is angled slightly so that it can catch the sunlight and reflect it:

Mica is formed in sheets rather than crystals and a stack of mica sheets is called a 'book' as shown in the following photograph of the mica I am holding. If one looks carefully on the right-side of the specimen, parallel lines of the individual 'sheets' in the 'book' can be seen.

As a child, finding a piece of this shiny mica was like finding gold! I liked to sit in the dirt and dig with a stick, looking for mica and other 'pretty rocks' 😁